Unprecedented Mutual Separation of Nd–Pr by Selective Dissolution of Nd₂O₃ Directly from a Nd₂O₃–Pr₆O₁₁ Mixture into an Ionic Liquid Containing a Highly Fluorinated β-Diketone

Abstract

An unprecedented mutual separation between neodymium (Nd) and praseodymium (Pr) has been achieved with a maximum separation factor of 700, which is more than 350 times better than the conventional hydrometallurgical method by selective dissolution of hexagonal Nd 2 O 3 directly for a physical mixture of Nd 2 O 3 and Pr 6 O 11 into an ionic liquid (C 4 mim.NTf 2 ) phase containing a fluorinated β-diketone, hexafluoroacetylacetone (HFAA), within 15 min of the dissolution process (leaving cubic Pr 6 O 11 as an undissolved solid residue). The selectivity was induced from the differences in the respective lattice energies of Nd 2 O 3 and Pr 6 O 11 in synergy with the controlled release of H + by HFAA into the ionic liquid and subsequent stabilization of Nd 3+ by complexation with the enolate form of HFAA arising out of the keto−enol tautomerism of the β-diketone. The initial sintering of the precursor oxides was found to be mandatory for the removal of the protective hydroxide layer of Nd on an aged Nd 2 O 3 sample. The water content in the ionic liquid, the concentration of HFAA, the solid-to-liquid ratio, stirring (or mixing) time, the relative composition of the Nd 2 O 3 −Pr 6 O 11 mixture, dissolution temperature, etc., strongly influenced the dissolution characteristics and hence the separation efficiency. Nd dissolution was found to be exothermic, as also evident from calorimetric measurements, indicating an enhancement in temperature with an enthalpy of dissolution (ΔH) of ∼24.66 J/g.